Semiconductor device and method of manufacturing the same

ABSTRACT

An insulating film made of the same material as that of a gate insulating film is formed so as to cover one sidewall of a control gate on a conducting film for floating gate. By selectively removing the conducting film for floating gate with the insulating film as a mask, a floating gate is formed from the conducting film for floating gate, and a portion of the gate insulating film is exposed at the floating gate. A nitrogen introduced portion is formed by introducing nitrogen into the exposed portion of the gate insulating film. Then, the insulating film is removed to expose an upper surface of a lateral protrusion of the floating gate. An erase gate is formed so as to face the upper surface and a side surface of the lateral protrusion.

This nonprovisional application is based on Japanese Patent ApplicationNo. 2015-164789 filed on Aug. 24, 2015, with the Japan Patent Office,the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to semiconductor devices and methods ofmanufacturing the same.

Description of the Background Art

Examples of nonvolatile flash memories include a split gate memoryhaving a floating gate, a control gate, an erase gate and a select gate.Such a nonvolatile flash memory is disclosed, for example, in JapanesePatent Laying-Open No. 2009-44164.

In this publication, after the control gate is patterned, a spacer oxidelayer is formed on sidewalls of the control gate. With this spacer oxidelayer as a mask, the floating gate below the control gate and the spaceroxide layer is patterned. The spacer oxide layer is then removed.

In the manufacturing method of the above publication, a gate oxide layerbelow the floating gate is exposed when the floating gate is patterned.Thus, when the spacer oxide layer is removed after the patterning of thefloating gate, the exposed gate oxide layer is also simultaneouslyremoved. As a result, the gate oxide layer below the floating gateerodes in a lateral direction, causing a lateral notch to occur in thegate oxide layer. Due to this notch, a stacked structure of the floatinggate and the control gate is supported unstably on a semiconductorsubstrate by the gate oxide layer.

For this reason, if megasonic cleaning (ultrasonic vibration) is appliedin this state in order to effectively remove dirt and dust in acidstripping and acid cleaning steps, the stacked structure of the floatinggate and the control gate may collapse with respect to the semiconductorsubstrate.

SUMMARY OF THE INVENTION

The other problems and new features will become apparent from thedescription of the present specification and the accompanying drawings.

A method of manufacturing a semiconductor device of one embodimentmainly includes the following steps.

A first sidewall insulating film made of the same material as that of agate insulating film is formed so as to cover one sidewall of a controlgate on a conducting film for floating gate. By selectively removing theconducting film for floating gate with the first sidewall insulatingfilm as a mask, a floating gate is formed from the conducting film forfloating gate, and a portion of the gate insulating film is exposed atthe floating gate. A first nitrogen introduced portion is formed byintroducing nitrogen into the exposed portion of the gate insulatingfilm. After the first nitrogen introduced portion is formed, the firstsidewall insulating film is removed to expose an upper surface of alateral protrusion of the floating gate protruding laterally from aregion immediately below the control gate. An erase gate is formed so asto face the upper surface and a side surface of the lateral protrusion.

According to the one embodiment described above, a semiconductor devicein which the collapse of a stacked structure of a floating gate and acontrol gate can be prevented if megasonic cleaning is applied, and amethod of manufacturing the same can be realized.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view schematically showing the configuration of asemiconductor device in a first embodiment.

FIG. 2 is a plan view showing a portion in an enlarged manner of theconfiguration in a memory cell array of the semiconductor device shownin FIG. 1.

FIG. 3 is a schematic cross-sectional view along a line in FIG. 2.

FIG. 4 is a cross-sectional view schematically showing a first step of amethod of manufacturing the semiconductor device in the firstembodiment.

FIG. 5 is a cross-sectional view schematically showing a second step ofthe method of manufacturing the semiconductor device in the firstembodiment.

FIG. 6 is a cross-sectional view schematically showing a third step ofthe method of manufacturing the semiconductor device in the firstembodiment.

FIG. 7 is a cross-sectional view schematically showing a fourth step ofthe method of manufacturing the semiconductor device in the firstembodiment.

FIG. 8 is a cross-sectional view schematically showing a fifth step ofthe method of manufacturing the semiconductor device in the firstembodiment.

FIG. 9 is a cross-sectional view schematically showing a sixth step ofthe method of manufacturing the semiconductor device in the firstembodiment.

FIG. 10 is a cross-sectional view schematically showing a seventh stepof the method of manufacturing the semiconductor device in the firstembodiment.

FIG. 11 is a cross-sectional view schematically showing an eighth stepof the method of manufacturing the semiconductor device in the firstembodiment.

FIG. 12 is a cross-sectional view schematically showing a ninth step ofthe method of manufacturing the semiconductor device in the firstembodiment.

FIG. 13 is a cross-sectional view schematically showing a tenth step ofthe method of manufacturing the semiconductor device in the firstembodiment.

FIG. 14 is a cross-sectional view schematically showing an eleventh stepof the method of manufacturing the semiconductor device in the firstembodiment.

FIG. 15 is a cross-sectional view schematically showing a twelfth stepof the method of manufacturing the semiconductor device in the firstembodiment.

FIG. 16 is a cross-sectional view schematically showing a thirteenthstep of the method of manufacturing the semiconductor device in thefirst embodiment.

FIG. 17 is a cross-sectional view schematically showing a fourteenthstep of the method of manufacturing the semiconductor device in thefirst embodiment.

FIG. 18 is a cross-sectional view schematically showing a fifteenth stepof the method of manufacturing the semiconductor device in the firstembodiment.

FIG. 19 is a cross-sectional view schematically showing a sixteenth stepof the method of manufacturing the semiconductor device in the firstembodiment.

FIG. 20 is a cross-sectional view schematically showing a seventeenthstep of the method of manufacturing the semiconductor device in thefirst embodiment.

FIG. 21 is a cross-sectional view schematically showing an eighteenthstep of the method of manufacturing the semiconductor device in thefirst embodiment.

FIG. 22 is a cross-sectional view schematically showing a nineteenthstep of the method of manufacturing the semiconductor device in thefirst embodiment.

FIG. 23A is a cross-sectional view showing a first step of a method ofmanufacturing a semiconductor device in a comparative example.

FIG. 23B is a cross-sectional view showing a second step of the methodof manufacturing the semiconductor device in the comparative example.

FIG. 23C is a cross-sectional view showing a third step of the method ofmanufacturing the semiconductor device in the comparative example.

FIG. 24 is a cross-sectional view schematically showing a method ofmanufacturing a semiconductor device in a second embodiment.

FIG. 25 is a cross-sectional view schematically showing theconfiguration of the semiconductor device in the second embodiment.

FIG. 26 is a cross-sectional view schematically showing a first step ofa method of manufacturing a semiconductor device in a third embodiment.

FIG. 27 is a cross-sectional view schematically showing a second step ofthe method of manufacturing the semiconductor device in the thirdembodiment.

FIG. 28 is a cross-sectional view schematically showing a third step ofthe method of manufacturing the semiconductor device in the thirdembodiment.

FIG. 29 is a cross-sectional view schematically showing a fourth step ofthe method of manufacturing the semiconductor device in the thirdembodiment.

FIG. 30 is a cross-sectional view schematically showing a method ofmanufacturing a semiconductor device in a fourth embodiment.

FIG. 31 is a cross-sectional view schematically showing theconfiguration of the semiconductor device in the fourth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments are described below with reference to the drawings.

First Embodiment

First, each region disposed on a main surface of a semiconductorsubstrate in a semiconductor device of this embodiment is described withreference to FIG. 1.

As shown in FIG. 1, a semiconductor device SD in this embodiment is inthe form of a chip, for example. Alternatively, semiconductor device SDin this embodiment may be in the form of a wafer, or in the form of aresin-sealed package.

Semiconductor device SD in this embodiment mainly has, on the surface ofa semiconductor substrate SB, an I/O (Input/Output) region, an ANALOGregion, a RAM (Random Access Memory) region, a CPU (Central ProcessingUnit) region, a charge pump region, and a flash memory region, forexample.

In the flash memory region, a plurality of nonvolatile flash memorycells are arranged in columns.

Next, the configuration of each of the nonvolatile flash memory cells inthis embodiment is described with reference to FIGS. 2 and 3.

As mainly shown in FIG. 2, the nonvolatile flash memory cell of thisembodiment is a split gate memory cell having a floating gate FG, acontrol gate CG, an erase gate EG and a select gate SG.

As seen in plan view, control gate CG, erase gate EG and select gate SGextend in juxtaposition with one another. As used herein, “seen in planview” refers to a visual point from a direction orthogonal to the mainsurface of semiconductor substrate SB.

An insulating film CI is disposed between select gate SG and controlgate CG. Select gate SG and control gate CG are electrically isolatedfrom each other by insulating film CI. Insulating film CI and a tunnelinsulating film TI are disposed between control gate CG and erase gateEG. Control gate CG and erase gate EG are electrically isolated fromeach other by insulating film CI and tunnel insulating film TI.

A sidewall insulating film SW is formed on a sidewall of select gate SGopposite to the side where control gate CG is disposed. This sidewallinsulating film SW extends in juxtaposition and in contact with selectgate SG.

A plurality of floating gates FG are disposed below one control gate CG.The plurality of floating gates FG disposed below one control gate CGare separated and electrically isolated from one another.

In addition, an element isolation insulating film ESR is formed on themain surface of semiconductor substrate SB. This element isolationinsulating film ESR is an STI (Shallow Trench Isolation), for example.The STI has a trench formed in the main surface of semiconductorsubstrate SB, and a buried insulating film buried in the trench.

As mainly shown in FIG. 3, semiconductor substrate SB has the mainsurface. The nonvolatile flash memory cell is formed on this mainsurface. This memory cell has, as described above, floating gate FG,control gate CG, erase gate EG, and select gate SG. This memory cellfurther has, in addition to the gates mentioned above, a drain regionDR, a source region SR, gate insulating films GI1 to GI4, and tunnelinsulating film TI.

Drain region DR is formed in the main surface of semiconductor substrateSB. Drain region DR has a low-concentration impurity region LD and ahigh-concentration impurity region HD. Low-concentration impurity regionLD and high-concentration impurity region HD are in contact with eachother. Drain region DR thus has an LDD (Lightly Doped Drain) structure.

Source region SR is formed in the main surface of semiconductorsubstrate SB at a distance from drain region DR. Floating gate FG andselect gate SG are disposed on the main surface of semiconductorsubstrate SB lying between drain region DR and source region SR.

Gate insulating film GI1 (first gate insulating portion) is formedbetween floating gate FG and the main surface of semiconductor substrateSB. That is, gate insulating film GI1 is formed on the main surface ofsemiconductor substrate SB, and floating gate FG is formed on gateinsulating film GI1. Gate insulating film GI1 is made of silicon oxide,for example.

Gate insulating film GI2 (IL4) (third gate insulating portion) is formedbetween select gate SG and the main surface of semiconductor substrateSB. That is, gate insulating film GI2 is formed on the main surface ofsemiconductor substrate SB, and select gate SG is formed on gateinsulating film GI2. Gate insulating film GI2 is made of silicon oxide,for example.

Control gate CG is formed on floating gate FG with gate insulating filmGI4 interposed therebetween. Gate insulating film GI4 has a stackedstructure of, for example, a silicon oxide film OX1, a silicon nitridefilm NI, and a silicon oxide film OX2. A hard mask layer HM is formed oncontrol gate CG.

Floating gate FG has a lateral protrusion FGP protruding laterally froma region immediately below control gate CG toward source region SR.Lateral protrusion FGP has a convex corner portion CN1 at an upper endcloser to source region SR. Lateral protrusion FGP also has a concavecorner portion CN2 at an upper end of the base portion locatedimmediately below control gate CG.

Floating gate FG, gate insulating film GI4, control gate CG and hardmask layer HM form a gate stack structure. Insulating film CI isdisposed between this gate stack structure and select gate SG. Floatinggate FG and control gate CG of the gate stack structure are electricallyisolated from select gate SG by insulating film CI.

Insulating film CI and tunnel insulating film TI are disposed betweenthe above gate stack structure and erase gate EG. Floating gate FG andcontrol gate CG of the gate stack structure are electrically isolatedfrom erase gate EG by insulating film CI and tunnel insulating film TI.

Each of insulating films CI located on opposite sides of the above gatestack structure has a two-layer structure of, for example, a siliconoxide film and a silicon nitride film. The silicon oxide film forminginsulating film CI is located closer to the gate stack structure, andthe silicon nitride film is located farther from the gate stackstructure.

Erase gate EG is formed so as to face lateral protrusion FGP of floatinggate FG with tunnel insulating film TI interposed therebetween.Specifically, erase gate EG is formed on an upper surface and a sidesurface of lateral protrusion FGP which form convex corner portion CN1,with tunnel insulating film TI interposed therebetween. Erase gate EGfaces an upper surface and a side surface of floating gate FG which formconvex corner portion CN1. Erase gate EG is formed on the main surfaceof semiconductor substrate SB. Gate insulating film GI3 (second gateinsulating portion) is formed between semiconductor substrate SB anderase gate EG.

Gate insulating film GI3 has a portion made of a material different fromthat of gate insulating film GI1. Gate insulating film GI3 has aninsulating film IL1, an insulating film IL2, and an insulating film IL3.Insulating film IL1 has the same thickness as, and is adjacent to, gateinsulating film GI1. Insulating film IL1 extends to a region immediatelybelow floating gate FG. Insulating film IL1 is made of a materialdifferent from that of gate insulating film GI1. When gate insulatingfilm GI1 includes a portion made of silicon oxide, for example,insulating film IL1 is made of silicon oxynitride in which nitrogen hasbeen introduced into silicon oxide, for example.

Insulating film IL2 is formed between insulating film IL1 andsemiconductor substrate SB (source region SR). This insulating film IL2has a convex shape bulging toward semiconductor substrate SB. Insulatingfilm IL2 is made of a material different from that of insulating filmIL1, and is made of silicon oxide, for example.

Insulating film IL3 is formed between insulating film IL1 and erase gateEG. Insulating film IL3 is part of tunnel insulating film TI. Insulatingfilm IL3 (TI) is made of a material different from that of insulatingfilm IL1, and is made of silicon oxide, for example.

Sidewall insulating film SW is formed across select gate SG from thegate stack structure. Sidewall insulating film SW is made of siliconoxide, for example. Low-concentration impurity region LD of drain regionDR is located in a region immediately below this sidewall insulatingfilm SW.

Each of floating gate FG, select gate SG, control gate CG and erase gateEG is made of polycrystalline silicon having an impurity introducedtherein (hereinafter referred to as doped polysilicon), for example.

Two gate stack structures are disposed to sandwich therebetween oneerase gate EG. Two select gates SG are disposed to sandwich therebetweenan arrangement in which the gate stack structure, erase gate EG and thegate stack structure are juxtaposed to one another. Specifically, one oftwo select gates SG is disposed across one of the two gate stackstructures from erase gate EG. The other of two select gates SG isdisposed across the other of the two gate stack structures from erasegate EG.

Insulating film IL1 extends from one of two floating gates FG to theother of floating gates FG.

A liner insulating film LI is formed so as to cover the memory cellsdescribed above. An interlayer insulating film II is formed so as tocover this liner insulating film LI. A contact hole CH is formed ininterlayer insulating film II and liner insulating film LI. Contact holeCH penetrates interlayer insulating film II and liner insulating film LIand reaches high-concentration impurity region HD of drain region DR. Aconducting film CL is buried in this contact hole CH. A wiring layer(not shown) is formed on interlayer insulating film II, and iselectrically connected to drain region DR through conducting film CL.

Next, a method of manufacturing the semiconductor device of thisembodiment is described with reference to FIGS. 4 to 22.

As shown in FIG. 4, semiconductor substrate SB having the main surfaceis prepared. Gate insulating film GI1 made of silicon oxide is formed bya thermal oxidation process, for example, on the main surface ofsemiconductor substrate SB. A conducting film FG for floating gate madeof doped polysilicon, for example, is formed on this gate insulatingfilm GI1. Conducting film FG for floating gate is patterned by aphotolithography process and an etching process.

Then, gate insulating film GI4 having a three-layer structure of siliconoxide film OX1, silicon nitride film NI and silicon oxide film OX2, forexample, is formed on conducting film FG for floating gate. A conductingfilm CG for control gate made of doped polysilicon, for example, isformed on conducting film FG for floating gate, with this gateinsulating film GI4 interposed therebetween. Hard mask layer HM made ofsilicon nitride, for example, is formed on this conducting film CG forcontrol gate.

As shown in FIG. 5, a photoresist PR1 is applied onto hard mask layerHM. This photoresist PR1 is patterned by exposure, development and thelike. Hard mask layer HM is patterned with patterned photoresist PR1 asa mask. Then, photoresist PR1 is removed by acid stripping, ashing andthe like, for example.

As shown in FIG. 6, conducting film CG for control gate and gateinsulating film GI4 are patterned with patterned hard mask layer HM as amask. Further, a prescribed amount of an upper surface of conductingfilm FG for floating gate is removed. As a result, a trench portion TRis formed on the upper surface of conducting film FG for floating gate.Concave corner portion CN2 is formed at an end portion of trench portionTR.

As shown in FIG. 7, a photoresist PR2 is applied onto semiconductorsubstrate SB. This photoresist PR2 is patterned by exposure, developmentand the like. Exposed conducting film FG for floating gate is removedwith patterned photoresist PR2 and hard mask layer HM as a mask. Then,photoresist PR2 is removed by acid stripping, ashing and the like, forexample.

As shown in FIG. 8, insulating film CI is formed so as to cover gateinsulating film GI1 and hard mask layer HM. This insulating film CI isformed of a silicon oxide film and a silicon nitride film thereon, forexample.

Anisotropic etching is performed on the entire surface of thisinsulating film CI until the upper surface of each of hard mask layer HMand gate insulating film GI1 is exposed. As a result of this etching,insulating film CI remains only on the sidewalls of hard mask layer HM,control gate CG and the like.

As shown in FIG. 9, an insulating film SL1 is formed so as to cover gateinsulating film GI1, hard mask layer HM and insulating film CI. Thisinsulating film SL1 is formed such that it is made of the same materialas that of gate insulating film GI1, and is made of silicon oxide, forexample. Anisotropic etching is performed on the entire surface of thisinsulating film SL1 until the upper surface of each of hard mask layerHM and gate insulating film GI1 is exposed. As a result of this etching,insulating film SL1 (first sidewall insulating film) remains only on asidewall of insulating film CI. Remaining insulating film SL1 covers oneof the sidewalls of control gate CG over floating gate FG, withinsulating film CI interposed therebetween.

A photoresist PR3 is applied onto semiconductor substrate SB. Thisphotoresist PR3 is patterned by exposure, development and the like.Exposed conducting film FG for floating gate is selectively removed withpatterned photoresist PR3, hard mask layer HM and insulating films CI,SL1 as a mask.

As shown in FIG. 10, as a result of the selective removal of conductingfilm FG for floating gate described above, floating gate FG is formedfrom conducting film FG for floating gate. The gate stack structure offloating gate FG, gate insulating film GI4, control gate CG and hardmask layer HM is thus formed. In addition, as a result of the removal ofconducting film FG for floating gate described above, gate insulatingfilm GI1 is partially exposed at floating gate FG.

As a result, floating gate FG is formed with lateral protrusion FGPprotruding laterally from the region immediately below control gate CG.The upper end of this lateral protrusion FGP has convex corner portionCN1.

As shown in FIG. 11, nitrogen is introduced into the exposed portion ofgate insulating film GI1. A nitrogen introduced portion IL1 (firstnitrogen introduced portion) is thus formed at the exposed portion ofgate insulating film GI1. A material for this nitrogen introducedportion IL1 is silicon oxynitride, for example.

As the conditions for the nitrogen introduction, an average projectionrange Rp is between 8 nm and 30 nm, and implantation energy is between 1keV and 5 keV, for example. Under these conditions, a sufficient amountof nitrogen is introduced into a silicon oxide film having a thicknessof about between 10 nm and 20 nm, for example.

In addition, as the conditions for the nitrogen implantation describedabove, an implantation angle is 0 degree, and a dose amount is between1×10¹⁵ atoms/cm² and 5×10¹⁵ atoms/cm², for example. The aboveimplantation angle allows the implantation of nitrogen to a deepposition. The above dose amount allows an etching rate of nitrogenintroduced portion IL1 to be sufficiently slow with respect to ahydrofluoric acid (HF) solution used in a later step.

It is noted that during this nitrogen introduction, nitrogen is alsointroduced into the upper surface of insulating film SL1 to form anitrogen introduced portion NI1. A material for this nitrogen introducedportion NI1 is also silicon oxynitride, for example.

Then, photoresist PR3 is removed by acid stripping, ashing and the like,for example. If photoresist PR3 is removed by acid stripping, forexample, megasonic cleaning can be applied. Cleaning is then performed.Megasonic cleaning can be applied to this cleaning as well.

As shown in FIG. 12, as a result of the removal of photoresist PR3described above, the upper surface of gate insulating film GI1, theupper surface of hard mask layer HM and the like are exposed. Annealingis then performed. This annealing takes place so as to form siliconoxynitride of thermally strong bonding in nitrogen introduced portionIL1.

The above annealing is performed in a nitrogen atmosphere, for example,at a temperature of between 950° C. and 1050° C. for a period of between10 seconds and 60 seconds. Nitrogen introduced portion IL1 is formed toslightly dig into the region immediately below floating gate FG.

Then, insulating film SL1 and gate insulating film GI1 exposed at thegate stack structure are removed by wet etching using a chemicalsolution of hydrofluoric acid. During this etching, nitrogen introducedportion NI1 formed on the upper surface of insulating film SL1 is alsosimultaneously removed.

As shown in FIG. 13, as a result of the above wet etching, the uppersurface of lateral protrusion FGP of floating gate FG and convex cornerportion CN1 are exposed. Acid cleaning is performed in this state.Megasonic cleaning can be applied to this acid cleaning.

As shown in FIG. 14, an insulating film SL2 is formed on the mainsurface of semiconductor substrate SB so as to cover the gate stackstructure (which includes floating gate FG and control gate CG) andnitrogen introduced portion IL1. Then, a photoresist PR4 is applied ontoinsulating film SL2. This photoresist PR4 is patterned by exposure,development and the like.

Impurity (for example, arsenic) ions are implanted into the main surfaceof semiconductor substrate SB with this photoresist PR4 as a mask.Source region SR is thus formed in the main surface of semiconductorsubstrate SB.

Then, photoresist PR4 is removed by acid stripping, ashing and the like,for example. If photoresist PR4 is removed by acid stripping, forexample, megasonic cleaning can be applied.

As shown in FIG. 15, a photoresist PR5 is applied onto insulating filmSL2. This photoresist PR5 is patterned by exposure, development and thelike. Insulating film SL2 is removed by etching with patternedphotoresist PR5 as a mask. The portion of insulating film SL2 exposed atphotoresist PR5 is thus removed, exposing hard mask layer HM, insulatingfilm CI, lateral protrusion FGP of the floating gate, and nitrogenintroduced portion IL1.

Then, photoresist PR5 is removed by acid stripping, ashing and the like,for example. If photoresist PR5 is removed by acid stripping, forexample, megasonic cleaning can be applied.

As shown in FIG. 16, after photoresist PR5 is removed, acid cleaning isperformed. Megasonic cleaning can be applied to this acid cleaning.Then, annealing is performed for activating the impurity that has beenintroduced into source region SR.

After the above annealing, acid cleaning is performed. Megasoniccleaning can be applied to this acid cleaning. Then, tunnel insulatingfilm TI is formed so as to cover the gate stack structure. This tunnelinsulating film TI is formed so as to cover convex corner portion CN1 oflateral protrusion FGP of the floating gate. Annealing is thenperformed.

As shown in FIG. 17, as a result of the above annealing, semiconductorsubstrate SB is oxidized below insulating film IL1. Insulating film IL2made of silicon oxide, for example, is thus formed between insulatingfilm IL1 and semiconductor substrate SB. In addition, as a result of theabove annealing, the thickness of tunnel insulating film TI locatedimmediately above insulating film IL1 increases.

Then, a photoresist PR6 is applied onto tunnel insulating film TI. Thisphotoresist PR6 is patterned by exposure, development and the like.Tunnel insulating film TI and insulating film SL2 are removed by etchingwith patterned photoresist PR6 as a mask. Hard mask layer HM, insulatingfilm CI, and the main surface of semiconductor substrate SB are thusexposed.

Then, photoresist PR6 is removed by acid stripping, ashing and the like,for example. If photoresist PR6 is removed by acid stripping, forexample, megasonic cleaning can be applied.

As shown in FIG. 18, after photoresist PR6 is removed, acid cleaning isperformed. Megasonic cleaning can be applied to this acid cleaning.Then, gate insulating film GI2 made of silicon oxide, for example, isformed on the main surface of semiconductor substrate SB. Then, aconducting film COL made of doped polysilicon, for example, is formed soas to cover the entire surface. This conducting film COL is etched back.

As shown in FIG. 19, as a result of the above etchback, the uppersurface of conducting film COL is located lower than the upper surfaceof the gate stack structure (the upper surface of hard mask layer HM).Erase gate EG and select gate SG are thus formed from conducting filmCOL. Erase gate EG is formed so as to face the upper surface and theside surface of lateral protrusion FGP of floating gate FG, with tunnelinsulating film TI interposed therebetween. Select gate SG is formedacross floating gate FG from erase gate EG.

As shown in FIG. 20, a photoresist PR7 is applied. This photoresist PR7is patterned by exposure, development and the like. Select gate SG ispartially removed by etching with patterned photoresist PR6 as a mask.Then, photoresist PR6 is removed by acid stripping, ashing and the like,for example.

As shown in FIG. 21, impurity ions are implanted into the main surfaceof semiconductor substrate SB. Low-concentration impurity region LD isthus formed in the main surface of semiconductor substrate SB.

As shown in FIG. 22, sidewall insulating film SW is formed so as tocover the sidewall of select gate SG. Then, impurity ions are implantedinto the main surface of semiconductor substrate SB. High-concentrationimpurity region HD is thus formed in the main surface of semiconductorsubstrate SB. High-concentration impurity region HD andlow-concentration impurity region LD form drain region DR.

Then, liner insulating film LI, interlayer insulating film II,conducting film CL and the like are formed, to manufacture thesemiconductor device of this embodiment shown in FIG. 3.

Next, the operation of the semiconductor device of this embodiment isdescribed.

As shown in FIG. 3, during writing, for example, 0.5 V is applied todrain region DR, 1 V is applied to select gate SG, 10 V is applied tocontrol gate CG, 5 V is applied to erase gate EG, and 5 V is applied tosource region SR. As a result, electrons emitted from drain region DRare accelerated by a strong electric field in a channel region betweendrain region DR and source region SR, forming hot electrons in a highenergy state. The hot electrons are attracted by the high potentialapplied to control gate CG, and implanted into floating gate FG. Data isthus written into the memory cell. The state in which the electrons areretained in floating gate FG is a write state of the memory cell. Athreshold voltage of the memory cell increases due to the electronsbeing retained in floating gate FG.

During erasure, for example, 0 V is applied to drain region DR, 0 V isapplied to select gate SG, 0 V is applied to control gate CG, 11 V isapplied to erase gate EG, and 0 V is applied to source region SR. As aresult, the electrons stored in floating gate FG are attracted by thehigh potential applied to erase gate EG, and drawn into erase gate EG bythe tunneling phenomenon. Here, with convex corner portion CN1 formed atthe upper end of lateral protrusion FGP of floating gate FG, theelectric field is concentrated at this convex corner portion CN1.Accordingly, as indicated by an outlined arrow in FIG. 3, the electronsin floating gate FG are preferentially drawn into erase gate EG throughconvex corner portion CN1, thereby improving drawing efficiency of theelectrons from floating gate FG. The electrons retained in floating gateFG are thus drawn out, causing erasure of the data held in the memorycell. The threshold voltage of the memory cell decreases due to theelectrons being drawn out of floating gate FG.

During reading, for example, 1.0 V is applied to drain region DR, 1.8 Vis applied to select gate SG, 1.8 V is applied to control gate CG, 0 Vis applied to erase gate EG, and 0 V is applied to source region SR. Inthis manner, when the memory cell is in an erase state, the thresholdvoltage of the memory cell is low and thus a read current flows. Incontrast, when the memory cell is in a write state, the thresholdvoltage of the memory cell is high and thus very little read currentflows. By detecting the magnitude of the read current, it can be readwhether or not the memory cell is in a write state.

Next, the function and effect of this embodiment is described incomparison with a comparative example shown in FIGS. 23A to 23C.

As shown in FIG. 23A, it is assumed that after the step of FIG. 10, wetetching for removing insulating film SL1 is performed without theintroduction of nitrogen into gate insulating film GI1.

In this case, as shown in FIG. 23B, gate insulating film GI1 erodes in alateral direction to a portion immediately below floating gate FG due tothe above wet etching. A lateral notch thus occurs in gate insulatingfilm GI1. Due to this notch, the gate stack structure having floatinggate FG and control gate CG is supported unstably by gate insulatingfilm GI1 on semiconductor substrate SB.

For this reason, if megasonic cleaning (ultrasonic vibration) is appliedin order to effectively remove dirt and dust in acid stripping and acidcleaning steps, the gate stack structure having floating gate FG andcontrol gate CG may collapse with respect to semiconductor substrate SBas shown in FIG. 23C.

In contrast, in this embodiment, nitrogen is introduced into gateinsulating film GI1 as shown in FIG. 11. Thus, the material for nitrogenintroduced portion IL1 is different from the material for insulatingfilm SL1. Accordingly, nitrogen introduced portion IL1 is less likely tobe removed even after insulating film SL1 is removed by wet etching inthe steps shown in FIGS. 12 to 13. Thus, the lateral notch in gateinsulating film GI1 in the region immediately below floating gate FG canbe reduced. As a result, the gate stack structure is less likely tocollapse if megasonic cleaning is applied.

If the material for nitrogen introduced portion IL1 is siliconoxynitride and the material for insulating film SL1 is silicon oxide,for example, it is well known that silicon oxynitride is resistant to ahydrofluoric acid (HF)-based solution. For example, the etching rate ofsilicon oxynitride containing nitrogen of about 1×10²¹ atoms/cm³ isapproximately 20% of the etching rate of silicon oxide.

If the time for treatment with a hydrofluoric acid-based solution forremoving 20 nm of insulating film SL1 is set (1 minute and 12 seconds)including an overetch time of 20%, gate insulating film GI1 having athickness of about 10 nm will be completely removed in the abovecomparative example, resulting in a lateral notch having a size ofbetween 14 and 24 nm. In contrast, when nitrogen introduced portion IL1is silicon oxynitride as in this embodiment, the etching rate isextremely reduced, and the lateral notch is also reduced to a maximum ofabout 4.8 nm.

Moreover, in this embodiment, the lateral notch in gate insulating filmGI1 can be reduced with the addition of a small number of steps.Specifically, only three steps should be added, namely, the step ofintroducing nitrogen utilizing photoresist PR3 (FIG. 10) which exists inthe comparative example as well, the step of forming silicon oxynitrideof thermally strong bonding by annealing thereafter, and the cleaningstep added to the above annealing step. In this manner, the steps ofthis embodiment are compatible with the manufacturing steps of thecomparative example.

Furthermore, in this embodiment, insulating film IL1 made of siliconoxynitride, for example, exists between erase gate EG and semiconductorsubstrate SB as shown in FIG. 3. Thus, the insulation between erase gateEG and semiconductor substrate SB as indicated by a black arrow in FIG.3 is improved as compared to the case where only silicon oxide existsbetween erase gate EG and semiconductor substrate SB. This is becausethe physical thickness of the insulating film between erase gate EG andsemiconductor substrate SB increases since insulating film IL1 itselfmade of silicon oxynitride remains, and because the dielectric constantincreases to increase capacity since insulating film IL1 containsnitrogen.

Second Embodiment

A method of manufacturing a semiconductor device of this embodimentincludes similar steps to those of the first embodiment shown in FIGS. 4to 13. In this embodiment, after insulating film SL1 (first sidewallinsulating film) is removed as shown in FIGS. 12 to 13, nitrogenintroduced portion IL1 (first nitrogen introduced portion) is removed byanisotropic etching until the main surface of semiconductor substrate SBis exposed as shown in FIG. 24. Then, the manufacturing method of thisembodiment includes similar steps to those of the first embodiment shownin FIGS. 14 to 22. A semiconductor device shown in FIG. 25 is thusmanufactured.

As shown in FIG. 25, the semiconductor device manufactured according tothis embodiment does not have nitrogen introduced portion IL1 betweenerase gate EG and semiconductor substrate SB. Accordingly, onlyinsulating films IL2 and IL3 made of silicon oxide, for example, existbetween erase gate EG and semiconductor substrate SB.

The configuration of this embodiment is otherwise substantially the sameas the configuration of the first embodiment, and thus the same elementsare designated by the same characters and description thereof will notbe repeated.

Again in this embodiment, as shown in FIGS. 12 to 13, nitrogenintroduced portion IL1 exists during the removal of insulating film SL1.Accordingly, as in the first embodiment, nitrogen introduced portion IL1is less likely to be removed even after insulating film SL1 is removedby wet etching in the steps shown in FIGS. 12 to 13. Thus, the lateralnotch in gate insulating film GI1 in the region immediately belowfloating gate FG can be reduced. As a result, the gate stack structureis less likely to collapse if megasonic cleaning is applied.

Third Embodiment

A method of manufacturing a semiconductor device of this embodimentincludes similar steps to those of the first embodiment shown in FIGS. 4to 14. After source region SR is formed in FIG. 14, in this embodiment,nitrogen is introduced into insulating film SL2 as shown in FIG. 26.

As the conditions for the nitrogen introduction, average projectionrange Rp is between 50 nm and 90 nm, and implantation energy is between8 keV and 15 keV, for example. As the conditions for the nitrogenimplantation described above, an implantation angle is 0 degree, and adose amount is between 1×10¹⁵ atoms/cm² and 5×10¹⁵ atoms/cm², forexample.

As a result, a nitrogen introduced portion NL1 (partial nitrogenintroduced portion) and a nitrogen introduced portion NL2 are formed ina portion of insulating film SL2. Nitrogen introduced portion NL1 isformed to be located on the upper surface of nitrogen introduced portionIL1, and also to be located on the side surface of lateral protrusionFGP of floating gate FG. Nitrogen introduced portion NL2 is formed to belocated in a portion of insulating film SL2 on hard mask layer HM and oninsulating film CI. These nitrogen introduced portions NL1 and NL2 aresilicon oxynitride, for example. Then, photoresist PR4 is removed byacid stripping, ashing and the like, for example. If photoresist PR4 isremoved by acid stripping, for example, megasonic cleaning can beapplied.

As shown in FIG. 27, as a result of the removal of photoresist PR4described above, a portion of insulating film SL2 that has been coveredwith photoresist PR4 is exposed. Annealing is then performed. Thisannealing takes place so as to form silicon oxynitride of thermallystrong bonding in nitrogen introduced portions NL1 and NL2. The aboveannealing is performed in a nitrogen atmosphere, for example, at atemperature of between 950° C. and 1010° C. for a period of between 10seconds and 60 seconds.

As shown in FIG. 28, a photoresist PR8 is then applied. This photoresistPR8 is patterned by exposure, development and the like. Wet etching isperformed with patterned photoresist PR8 as a mask. As a result of thiswet etching, a portion of insulating film SL2 exposed at photoresist PR8other than the portions of nitrogen introduced portions NL1 and NL2 isselectively removed.

Then, photoresist PR8 is removed by acid stripping, ashing and the like,for example. If photoresist PR8 is removed by acid stripping, forexample, megasonic cleaning can be applied. Then, nitrogen introducedportions NL1 and NL2 are removed.

As shown in FIG. 29, as a result of the removal of nitrogen introducedportions NL1 and NL2 described above, the side surface of lateralprotrusion FGP of floating gate FG is exposed.

Then, similar steps to those of the first embodiment shown in FIGS. 16to 22 are performed. A semiconductor device having a similarconfiguration to that of the second embodiment shown in FIG. 25 is thusmanufactured.

In this embodiment, during the removal of insulating film SL2 shown inFIGS. 27 to 28, in addition to nitrogen introduced portion IL1 beinglocated lateral to gate insulating film GI1, the side surface of lateralprotrusion FGP of floating gate FG is covered with nitrogen introducedportion NL1. Accordingly, the occurrence of a lateral notch in gateinsulating film GI1 in the region immediately below floating gate FG canbe reduced during the removal of insulating film SL2.

Moreover, in this embodiment, also during the removal of photoresist PR8shown in FIG. 28, in addition to nitrogen introduced portion IL1 beinglocated lateral to gate insulating film GI1, the side surface of lateralprotrusion FGP of floating gate FG is covered with nitrogen introducedportion NL1. Accordingly, the occurrence of a lateral notch in gateinsulating film GI1 in the region immediately below floating gate FG canbe reduced also during the removal of photoresist PR8.

Fourth Embodiment

A method of manufacturing a semiconductor device of this embodimentincludes similar steps to those of the first embodiment shown in FIGS. 4to 7. Then, in this embodiment, as shown in FIG. 30, nitrogen isintroduced into gate insulating film GI1 exposed at photoresist PR2 andthe gate stack structure (that is, a portion of gate insulating film GI1exposed at conducting film FG for floating gate at the other sidewallside of control gate CG).

As the conditions for the nitrogen introduction, average projectionrange Rp is between 8 nm and 30 nm, and implantation energy is between 1keV and 5 keV, for example. In addition, as the conditions for thenitrogen implantation described above, an implantation angle is 0degree, and a dose amount is between 1×10¹⁵ atoms/cm² and 5×10¹⁵atoms/cm², for example.

As a result, a nitrogen introduced portion IL3 (second nitrogenintroduced portion) is formed in a portion of gate insulating film GI1.This nitrogen introduced portion IL3 is silicon oxynitride, for example.Then, photoresist PR2 is removed by acid stripping, ashing and the like,for example.

Then, the manufacturing method of this embodiment includes similar stepsto those of the first embodiment shown in FIGS. 8 to 22. A semiconductordevice shown in FIG. 31 is thus manufactured.

As shown in FIG. 31, the configuration of the semiconductor devicemanufactured according to this embodiment is different from theconfiguration of the first embodiment shown in FIG. 3 in terms of theconfiguration of gate insulating film GI2 located immediately belowselect gate SG.

As shown in FIG. 31, in this embodiment, gate insulating film GI2 has atwo-layer structure of nitrogen introduced portion IL3 and insulatingfilm IL4. Nitrogen introduced portion IL3 is made of silicon oxynitride,for example. Insulating film IL4 is in contact with the upper surface ofnitrogen introduced portion IL3, and is made of silicon oxide, forexample. Gate insulating film GI2 thus has a portion made of a materialdifferent from that of gate insulating film GI1 in that gate insulatingfilm GI2 includes nitrogen introduced portion IL3.

The configuration of this embodiment is otherwise substantially the sameas the configuration of the first embodiment, and thus the same elementsare designated by the same characters and description thereof will notbe repeated.

In this embodiment, as shown in FIG. 31, nitrogen introduced portionsIL1 and IL3 exist at opposite sides of gate insulating film GI1.Accordingly, nitrogen introduced portions IL1 and IL3 are less likely tobe removed even after insulating film SL1 is removed by wet etching inthe steps shown in FIGS. 12 to 13. Thus, the lateral notch at oppositesides of gate insulating film GI1 in the region immediately belowfloating gate FG can be reduced. As a result, the gate stack structureis even less likely to collapse if megasonic cleaning is applied.

While the material for each of gate insulating film GI1 and insulatingfilm SL1 has been described as silicon oxide in the above embodiments,the material for each of gate insulating film GI1 and insulating filmSL1 may be a high-dielectric constant insulating material (for example,hafnium oxide, tantalum oxide).

In addition, while the material for each of nitrogen introduced portionsIL1 and IL3 has been described as silicon oxynitride, if the materialfor each of gate insulating film GI1 and insulating film SL1 is ahigh-dielectric constant insulating material (for example, hafniumoxide, tantalum oxide), nitrogen may be introduced into thehigh-dielectric constant insulating material.

In addition, the etching rate of insulating film SL1 may be faster thanthat of gate insulating film GI1.

Although the invention made by the present inventors has beenspecifically described based on the embodiments, it goes without sayingthat the present invention is not limited to the embodiments describedabove, but can be modified in various ways within the gist of thepresent invention.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

What is claimed is:
 1. A method of manufacturing a semiconductor device,comprising the steps of: forming a gate insulating film on a mainsurface of a semiconductor substrate; forming a conducting film forfloating gate on the gate insulating film; forming a control gate on theconducting film for floating gate with a first insulating filminterposed therebetween; forming a first sidewall insulating film madeof the same material as that of the gate insulating film, so as to coverone sidewall of the control gate on the conducting film for floatinggate; by selectively removing the conducting film for floating gate withthe first sidewall insulating film as a mask, forming a floating gatefrom the conducting film for floating gate, and exposing a portion ofthe gate insulating film at the floating gate; forming a first nitrogenintroduced portion by introducing nitrogen into the exposed portion ofthe gate insulating film; after forming the first nitrogen introducedportion, removing the first sidewall insulating film to expose an uppersurface of a lateral protrusion of the floating gate protrudinglaterally from a region immediately below the control gate; and formingan erase gate so as to face the upper surface and a side surface of thelateral protrusion.
 2. The method of manufacturing a semiconductordevice according to claim 1, further comprising the step of, afterremoving the first sidewall insulating film, removing the first nitrogenintroduced portion by anisotropic etching until the main surface of thesemiconductor substrate is exposed.
 3. The method of manufacturing asemiconductor device according to claim 1, further comprising the stepsof: after exposing an upper surface of the floating gate, forming asecond insulating film so as to cover the floating gate, the controlgate and the first nitrogen introduced portion; after forming the secondinsulating film, forming a source region by implanting an impurity intothe semiconductor substrate; and after forming the source region,forming a partial nitrogen introduced portion by introducing nitrogeninto the second insulating film located on the side surface of thelateral protrusion of the floating gate.
 4. The method of manufacturinga semiconductor device according to claim 3, further comprising the stepof removing a portion of the second insulating film other than thepartial nitrogen introduced portion.
 5. The method of manufacturing asemiconductor device according to claim 4, further comprising the stepof, after removing the portion of the second insulating film other thanthe partial nitrogen introduced portion, removing the partial nitrogenintroduced portion and the first nitrogen introduced portion.
 6. Themethod of manufacturing a semiconductor device according to claim 1,further comprising the step of, after forming the control gate andbefore forming the first sidewall insulating film, forming a secondnitrogen introduced portion by introducing nitrogen into a portion ofthe gate insulating film exposed at the conducting film for floatinggate at the other sidewall side of the control gate.
 7. The method ofmanufacturing a semiconductor device according to claim 1, wherein thestep of forming the erase gate includes the step of forming a selectgate across the floating gate from the erase gate.